The present invention relates to an inversion development controller for use in an image forming apparatus such as a copying machine.
In a copying machine with a conventional development system using a two component developer, development has been performed by first exposing the surface of a positively charged photosensitive drum to form a latent image on the drum surface. Then negatively charged toners and positively charged carriers are made to adhere onto a non-exposed region of the latent image portion on the drum surface.
In a copying machine with an inversion development system using a two component developer, the surface of a photosensitive drum is negatively charged. In an inversion development system, negatively charged toners are made to adhere to an exposed portion of the negatively charged drum surface which has zero voltage.
The inversion development system described above is shown in prior art FIG. 1. A negative voltage of about -700 volts to about -800 volts is applied to a portion of the surface of photosensitive drum 2 by a charger 1. This creates a negatively charged portion on the surface of photosensitive drum 2.
As photosensitive drum 2 rotates, the negatively charged portion is positioned opposite exposing rod lens array 3 for exposure. Exposure creates a latent image of zero volts on the negatively charge portion of photosensitive drum 2. After exposure, photosensitive drum 2 rotates further, and the exposed negatively charged portion on the surface of photosensitive drum 2 arrives at a position opposite development roller 41. At this time, a bias voltage of about -400 volts is applied to development roller 41, causing negatively charged toners on the development roller 41 to be repulsed toward (fly) and adhere to the exposed portion of photosensitive drum 2 having zero voltage.
It is desirable that the bias voltage be applied to the development roller 41 at the same time that the exposed negatively charged portion on the surface of photosensitive drum 2 reaches the position opposite development roller 41; however, it is difficult to control such timing. When the timing is off, the bias voltage may be applied either before or after the exposed negatively charged portion has reached a position opposite development roller 41.
FIG. 2A shows a situation in which the bias voltage is applied before the exposed negatively charged portion reaches the position opposite development roller 41. The portion of photosensitive drum 2 which is positioned opposite development roller 41 has a surface voltage greater than the bias voltage of development roller 41. This causes toner particles to fly from development roller 41, and adhere to the portion of photosensitive drum 2 positioned opposite development roller 41. The voltage difference between development roller 41 and the portion of photosensitive drum 2 positioned opposite development roller 41 exceeds an allowable voltage difference range as shown in FIG. 2B. The allowable voltage difference range shown in FIG. 2B is the voltage difference range in which the bias voltage can differ from the surface voltage of the portion of photosensitive drum 2 opposite development roller 41 without causing toners or carrier particles to fly.
FIG. 3A shows the situation in which the bias voltage applied to development roller 41 is applied after the exposed negatively charged portion of photosensitive drum 2 reaches a position opposite development roller 41. The portion of photosensitive drum 2 which is opposite development roller 41 has a voltage less than the bias voltage applied to development roller 41. When this occurs, positively charged carriers are attracted onto the surface of photosensitive drum 2. As shown in FIG. 3B, the voltage difference between the bias voltage (the voltage of development roller 41) and the surface voltage of the portion of photosensitive drum 2 opposite development roller 41 exceeds the allowable voltage difference range and carriers fly.
A proposed solution to the above-mentioned problems depicted in FIGS. 2 and 3 is to gradually apply the bias voltage. This solution has the disadvantage that if the timing of the bias voltage is incorrect, the resulting voltage difference between the development roller 41 and the surface of photosensitive drum 2 opposite development roller 41 exceeds the allowable voltage difference range. However, in this situation, neither toners nor carriers adhere to the photosensitive drum; instead, the toners or carriers scatter.